In a recent pilot study, we measured long-term pulmonary clearance in a small group of smokers and non-smokers; a one-time inhalation of magnetic dust was used as a harmless tracer. This dust was cleared much slower in smokers than in non-smokers; after one year, five times as much dust was retained in the lungs of smokers as in non-smokers. We here will further investigate this result with a larger study. A magnetic tracer dust will again be inhaled, and clearance will be measured with more elaborate techniques; six heavy and three ex-heavy smokers (of high tar-nicotine cigarettes), three heavy smokers (low tar-nicotine), and three non-smokers will be studied. Our purpose is to determine if this high retention (and attendant risk) is a function of nicotine-tar content, and if it declines to non-smoker levels in ex-smokers. In the second part of this study, magnetic dust content will be measured in the lungs of a number of heavy smokers and non-smokers, all with long exposure to urban dust. Also, the same measurements will be made of a number of smoking and ten non-smoking arc welders. The purpose is to determine if the higher dust retention seen in smokers also occurs in individuals experiencing normal urban living or during occupational exposure.